Macaques which had been immunized as described in previous reports were challenged with 10 AID50 of infectious SIVDnef on 4/15/93. We wanted to determine whether immunization with a combination of SIV recombinant proteins prevented infection with this attenuated virus. Fourteen out of 22 macaques became infected with this virus; however, one macaque belonging to a control group was negative for virus isolation, therefore questioning the viability of the viral inoculum thatwas used. All animals, plus four naive macaques, were re-inoculated with 100 AID50 of the same SIVDnef on 9/30/93 (week 24). The outcome was very variable some macaques that had been virus-isolation positive, and later turned negative, were positive again indicating that they didnUt resist the new challenge; on the contrary, there were monkeys that were positive first, negative later, and resisted the second challenge. Most of the monkeys that were negative for the first inoculation became positive for virus isolation; however, one of the naive controls remained negative. Finally, we decided to study whether this combination of recombinant subunit and live-attenuated vaccines prevented infection of macaques against a highly virulent biological isolate. Animals were challenged on 5/19/94, 33 weeks after the second SIVDnef inoculation, with 100 AID50 of SIVmac251; a new group of four naive macaques was added. Twenty two out of 30 macaques tested positive for SIVmac251 virus. There were two animals that were positive for SIVDnef, three that showed dual infection, and three that were negative for any virus. The experiment was terminated one year after the challenge with SIVmac251, at which time 7 monkeys had died of SAIDS; two belonged to the naive control group, two were exposed to SIVDnef but were negative the day of challenge, and three had a mixed infection.